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Message-ID: <20160306010843.GQ9349@brightrain.aerifal.cx> Date: Sat, 5 Mar 2016 20:08:43 -0500 From: Rich Felker <dalias@...c.org> To: Pedro Giffuni <pfg@...eBSD.org> Cc: musl@...ts.openwall.com Subject: Re: FreeBSD's Google Summer of Code 2016 On Sat, Mar 05, 2016 at 07:59:35PM -0500, Pedro Giffuni wrote: > > > On 03/05/16 19:25, Rich Felker wrote: > >On Sat, Mar 05, 2016 at 07:14:34PM -0500, Pedro Giffuni wrote: > >> > >> > >>On 03/05/16 18:32, Rich Felker wrote: > >>>On Sat, Mar 05, 2016 at 05:41:25PM -0500, Pedro Giffuni wrote: > >>>>First of all, great to hear there is interest on the musl side too. > >>>> > >>>>I think the biggest precedent of porting linux-oriented C libraries > >>>>came from Debian's kFreeBSD. We accomodated a little by for them > >>>>by defining __FreeBSD_kernel__ in sys/param.h. > >>>> > >>>>While using the optional linux-abi futex in FreeBSD could be an option, > >>>>it is not really the cleanest option. The Debian guys did a port of > >>>>NPTL using regular pthreads: > >>>> > >> > >>Of course I ahould have meant "based on regular FreeBSD kernel services". > >> > >>>>http://thread.gmane.org/gmane.linux.debian.ports.bsd/11702 > >>>> > >>>>I am certain this will require more research but it would be useful > >>>>for other ports as well. > >>> > >> > >>We could ask Petr Salinger for the details, but I am pretty sure > >>FreeBSD has the required functionality natively. > >> > >>>Glibc/NPTL has a lot of what I'd call "gratuitous abstraction" (like > >>>the lll stuff) in their pthread primitives which makes this > >>>"possible". I call it gratuitous because it's really really hard to > >>>achieve correct implementations of the pthread sync primitives that > >>>don't have serious corner-case bugs, and it's unlikely that their > >>>abstractions actually suffice to make correct alternate > >>>implementations. > >>> > >>>musl does not have any such abstraction. We require a compare-and-swap > >>>operation or equivalent on which arbitrary atomic operations can be > >>>constructed, a futex or equivalent operation that's roughly > >>>while(*addr==expected) sleep(), and implement all the sync primitives > >>>just once on top of these. > >>> > >> > >>I am not a threading expert (or even a CS guy), but it sounds like > >>mutex(9) with condvar(9) would do [1]: > > > >No, they don't satisfy the needs of musl; they have their own > >additional storage requirements and are probably not AS-safe. It might > >be possible to use them to implement a userspace-emulated futex queue > >(only if they are AS-safe), but I don't see a way to extend that to > >the process-shared case. > > > > OK, it looks like sema(9) may be nearer (and also simpler but slower). > For the process-shared case libthr(2) uses the stuff in sys/utmx.h, > shich should be looked at but it is not documented[1]. > > Luckily Ed, the developer that would be mentoring the project, knows > this stuff better than I do. OK, well let's see if there are any proposals and what ideas people come up with. Rich
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